There is known a technology of using a low-pass filter (LPF) to remove noise of a high-frequency component included in an image.
An example of this technology, there is an edge-preserving type image processing apparatus. With respect to each of LPFs used in the edge-preserving type image processing apparatus, a different range of level values is set as a passage range of level values of pixels enabled to pass through the LPF. The edge-preserving type image processing apparatus inputs an image to the LPFs, and smooths the image by performing convolution operation between a level value of each pixel and level values of pixels surrounding the pixel with respect to each of output results output from the LPFs. Then, the edge-preserving type image processing apparatus adopts, out of results of the smoothing that the respective output results from the LPFs are smoothed, a result of the smoothing obtained from the output result from an LPF that admits a level value of a pixel included in the input image as the passage range as a level value of an output pixel. Thus, the edge-preserving type image processing apparatus removes a high-frequency component while preserving the edge of the input image.
As another example, there is a reduced-image using type image processing apparatus that generates an output image by synthesizing respective results of smoothing obtained by LPFs from a reduced image of a previously-input frame in a moving image sequence which has been input to the apparatus and a newly-input frame. This reduced-image using type image processing apparatus uses a reduced image of a previous frame at the time of smoothing an input image, and therefore can reduce a processing amount of each LPF while retaining the advantages of the above-described edge-preserving type image processing apparatus.
As still another example, there is a complex type noise reduction device that uses a frame-cyclic type noise reduction device and a non-cyclic type filter noise reduction device. For example, when any image motion between frames has not been detected, and a signal level of an image signal input to the device is low, the complex type noise reduction device performs noise reduction using the frame-cyclic type noise reduction device. Furthermore, when an image motion between frames has been detected, the complex type noise reduction device performs noise reduction using the non-cyclic type filter noise reduction device. In this manner, the complex type noise reduction device uses an optimal noise reduction method according to the presence or absence of a motion in an image signal and the strength of the image signal.    Patent document 1: International Publication Pamphlet No. WO 2008/020487    Patent document 2: International Publication Pamphlet No. WO 2009/060633    Patent document 3: Japanese Laid-open Patent Publication No. 2002-010106
However, the above-described conventional technologies have a problem that the memory capacity is enlarged when flicker of a low-frequency component occurring between frames of moving images, i.e., temporal low-frequency noise is reduced.
For example, in the case of the reduced-image using type image processing apparatus, noise of a high-frequency component which is prone to attract people's attention is removed, and as a result, flicker of a low-frequency component occurring between frames of moving images becomes prominent. Furthermore, in the case of the complex type noise reduction device, an image acquired from an image signal is averaged with previously-acquired multiple images. To perform the averaging, previous temporally-successive frames are accumulated, and therefore, a large-capacity memory has to be adopted.